1. Technical Field
The present invention relates to drilling of hydrocarbon wells. In particular, the invention relates to a method and a drilling control system for providing risk reduction and improved efficiency of a drilling process.
2. Background Information
When drilling a hydrocarbon well, such as a subsea well, it is known to operate the drilling equipment through a computerized drilling system. The drilling operator controls the various parameters of the process using control devices such as joysticks, throttles or switches. The control devices are connected to equipment controls, such as a controller for the rotary table.
When drilling such a well, one wishes to drill the well as efficiently as possible, with regards to time, cost and safety, while at the same time avoiding doing any damage to the formation being drilled, which may contain producible gas and oil reservoirs. To achieve this one must adapt the drilling process to drilling of the well in question. This has been the case for the history of drilling of oil and gas wells.
Systems are known which monitor drilling control parameters in order to prevent damage to the drilling equipment, for instance to the drill bit or tubular (drill string, casing or liner). Such control parameters can include drill string velocity, drill string torque, drill string RPM, hook load, WOB, pump flow rate, and choke opening and choke pump flow rate. They may automatically generate an alarm if a critical situation is detected.
The challenges in controlling the drilling process are not new, but drilling of oil and gas wells is becoming more and more of a challenge. Known reservoirs are being depleted, leading to problems with both formation stability and narrowing pressure windows. Expanding areas of exploration and production, including increasing activities in arctic and deep sea/deep reservoir areas, are generating new demands on safety and accuracy of drilling process control.
Patent publication U.S. Pat. No. 7,172,037 (Baker Huges Inc.) describes a system for optimizing a drilling process by providing optimized parameters to the driller or drilling control system. Patent publication U.S. Pat. No. 6,662,110 also regards a system for optimization of a drilling process as well as for protection of well drilling systems.
Patent publication U.S. Pat. No. 6,968,909 (Schlumberger) describes a downhole drilling system that is based on running scripts for various drilling steps and drilling conditions. For instance, a tripping script is run for tripping of the drill string. Thus, with this system, the drilling is performed on “autopilot”, for as long as the system recognizes what is taking place (“diagnostic” (316) and “manual control” (320) in FIG. 3).
This automated system collects downhole and surface measurements to continually update drilling process models and to calculate optimized drilling parameters as well as operating limits. In addition, it contains automated analysis of the drilling conditions, which can result in running of a remedying script if an undesired condition is detected.
It is, however, desirable to perform drilling operations manually, in the sense of controlling the drilling equipment, such as the top drive/the rotary table, mud pumps, and the winch drive (drawworks) with suitable interface means, such as a joystick, without the risk of damaging the well due to human error. The present invention provides a novel solution to this task.
Furthermore, current systems for optimization of drilling parameters work independently, individually controlling one parameter or a set of parameters to enable optimization with respect to a particular mechanism. Full optimization with respect to individual mechanisms may be detrimental to other process mechanisms. As an example, fully optimized rate of penetration with respect to specific mechanical energy, through adjustment of WOB and RPM may lead to cuttings build-up issues if the pump rate is not adjusted accordingly, which is further constrained by the existing formation geopressures. Intelligent coordination between different input to optimization and given constraints is desirable to ensure that the overall process is optimal.
A new methodology has been developed to fulfil the requirements described above. The overall objective of this method is to maintain the functioning of the drilling machinery within safeguards accounting for both the machine limitations but also the wellbore limits. In addition, automatic triggering of corrective actions can assist in maintaining the well integrity in case of abnormal situations.
The goal of this methodology is not to completely automate part or the whole of the drilling process, but to apply continuously updated envelopes of protection. Therefore the operator has the freedom to operate the drilling machinery as he wishes, while he is given assistance in maintaining the drilling conditions within safe boundaries. This methodology is solely used by the drilling machinery operators.
The methodology provides direct machine control but can also provide early problem detection during the drilling process, so that the operator can decide on corrective actions, or alternatively trigger automatic actions in case of emergency to take advantage of the rapidity of computer controlled machine steering.
When determining preferable drilling control parameters today, such as ROP, WOB, applied drillstring torque and drilling fluid circulation rate, one takes into account such properties as the dimensions of the well, formation properties (e.g. stresses, geopressures, geothermal), the drillstring (e.g. bit type, material properties of string elements) and the drilling fluid (e.g. density, rheology). For updating of optimal parameters, analysis of well behavior during drilling of the well may be performed, where available data from sensors on the rig and downhole are applied, possibly together with results from active testing of the well. From such analysis permissible operational windows and process constraints may also be determined Such analysis is normally performed independently of the drilling operation on the rig.
Process constraints comprise machine limits, material limits and wellbore/formation limits Machine limits (e.g. maximum power of draw works engines) and material limits (e.g. maximum torque on drill string elements) are provided by the suppliers of the drilling machinery. Wellbore and formation limits may be determined by analysis of historical data from offset wells and survey data, and by active testing of the well (e.g. Leak Off Test/Formation Integrity Test to determine upper pressure bound). Such active tests are performed by the drilling crew on the rig.
Since all decision making is done by the operator based on availability of information, there will always be a time delay before any action is taken when undesired symptoms are observed. During this delay there is a great risk of the observed problem escalating and becoming more severe (e.g. pressure buildup).
Unexpected behavior occurring during drilling operations is today detected by the drilling crew with the aid of alarms. It is the drilling crew's task to interpret behavior and take appropriate mediating measures. The reaction taken depends on the experience of the crew, and various procedures may be used for the same type of incident. This is one of the major challenges of today's drilling process. The full know-how of the organization is not applied, and inappropriate mediating actions may cause loss of time, loss of production, and possibly loss of the well. Thus, as will appear from the description below, an advantageous embodiment of the invention comprises means for rapid remedial action.